1. Field of the Invention
The present invention relates to projection exposure apparatus for projecting the images of a pattern formed on a mask or reticle onto the photosensitive surface of a wafer and more particularly to an improved exposure apparatus capable of satisfactorily utilizing the same alignment mark formed on a wafer a plurality of times during alignment.
2. Description of the Prior Art
In a known exposure apparatus of the reduction projection type employing a reticle, for example, during alignment any diversion of the alignment reticle marks on a reticle from the alignment wafer mark on a wafer is measured and after the completion of the alignment the wafer is exposed at this alignment position. In this case, the pattern is imaged along with the reticle marks, on the wafer.
As a result, when the resist on the exposed wafer is developed and the wafer is then etched, the reticle marks are formed in the vicinity of the wafer mark and the wafer surface shape near the wafer mark is disturbed, thus giving rise to the danger of failing to use the wafer mark again as an alignment mark during the next alignment.
Thus, there have been known in the art a projection exposure apparatus so constructed that during each exposure the reticle mark is covered to prevent the reticle marks from being projected and printed on the wafer and a projection exposure apparatus so, designed that during each alignment a separate wafer mark formed at a different position is used. The former has the disadvantage of requiring the provision of a reticle mark covering device in the limited space above the reticle and complicating the construction of the apparatus and also the latter the disadvantage of requiring the provision of an increased space on the wafer for the formation of wafer marks and complicating the alignment operation for each wafer mark.